The proposed study seeks to further our understanding of cell division. Microtubules, the major structural components of the mitotic apparatus, or spindle, have been implicated in playing a major role in the generation of the mitotic movements. In the spindle, microtubules arise from specific microtubule organizing centers, the kinetochores of chromosomes and the centrosomes located at the mitotic poles. Recently, techniques have been defined bny which the protein dynein can be used to determine the structural polarity of microtubules to which the dynein bound, including those in th spindle, and experiments will now be conducted to determine the polarity of growth of the mitotic microtubules by using bound dynein to identify those regions of the microtubules assembled after decoration. Both the kinetochores and centrosomes serve as sites whch initiate the in vitro assembly of microtubules from dimers of the protein tubulin. Studies will now be carried out to assess whether these two microtubule organizing centers can also serve as sites of attachment of preformed microtubules. Not all of the microtubules within the kinetochore fiber connect the organelle to the centrosome, and detailed analyses of the structural polarity of all classes of microtubules within the kinetochore fiber will be undertaken. Finally, studies of the equatorial region of the spindle in which centrosomal microtubules from opposite poles overlap will be made, again suing dynein as a polarity marker. The extent of the region of overlap and spatial arrangements of the overlapping microtubules will be determined at various stages of mitosis, and the predictions of models of mitosis, suggesting the sliding of antiparallel microtubules in the overlap region, will be tested. Results of these studies will furter our knowledge of how the mitotic apparatus is established and then functions to segregate the chromosomes into daughter cells. It will thus provide potentially useful information for other studies of cells undergoing rapid and uncontrolled proliferation.